A prior art control sensor used for controlling a grader or a similar construction machine mechanically traces the guide cable and thereby generates signals of height and direction control indicating the relative position of the sensor to the guide cable. Thus, the prior art mechanical control sensor has to fulfill two functions, i.e. the generation of a height control signal for the vertical adjustment of a ground-working tool and the generation of a direction control signal with which, for example, a necessary correction of direction of a movement may be indicated to a dirver on a direction indicator unit. Attention has to be drawn to the fact that the height control signal must be generated with the highest precision possible to meet with today's demands of road construction. However, by nature the mechanical sensor proves to be susceptible to damages which may easily occur during rugged activities in the field of road construction. More and more functions are integrated into today's construction machinery to increase their efficiency. Therefore, the driver or operator of the construction machine has to control and supervise a great deal of operational stages. A relief for the driver of these perpetually occurring standard functions can only be achieved to some extent by the prior art control sensor, as it is not suitable to inform the operator of an excessive deviation of the direction. Also, the prior art control sensor may not recognize an abandonment of the guide cable due to an incorrect direction control of the construction machine through which the construction machine could do considerable damage.
It is known from DE-U-87 13 874 of the applicant to make use of a supersonic distance control device for the purpose of measuring the distance with respect to ground in the field of road construction. A supersonic distance gauging device of this kind is unsuitable for the purpose of generating a height control signal for indicating the distance of the distance gauging device to the guide cable with an accuracy adequate to the control of a construction machine as a lateral misalignment of the guide cable towards the distance gauging device would lead to an incorrect--indicating too great a height--height control signal, thus the man skilled in the present field restricts the use of this supersonic distance gauging device, while demanding a high standard of accurate control, to such purposes where the aim is the supersonic distance measurement of the distance of a surface to the sensor where such measuring errors cannot occur. Therefore, the only indicated field of application of this supersonic distance gauging device is the determination of distance to a surface, as mentioned in the DE-U.
U.S. Pat. No. 4,240,152 discloses an obstacle detecting means for vehicles where severe supersonic sensors are arranged parallel to the vehicle's bumper. These supersonic sensors serve the purpose of detecting obstacles, for example when the vehicle is reversing, which may be arranged in the "blind spot".
U.S. Pat. No. 4,623,032 discloses a supersonic distance measuring appliance for remote control vehicles which comprises two supersonic sensors to detect the position of an obstacle situated in the space in front of the vehicle.
DE-A-35 18 613 discloses an obstacle registration system to register the position of an obstacle by means of a plurality of supersonic sensors which can be differently controlled with regard to their phases.